Downhole drilling operations are generally performed at remote locations, and it is therefore desirable to reuse or recycle as many resources as possible on-site to avoid excessive environmental damage and reduce unnecessary transportation costs. In addition, any other means to reduce equipment, material, and transportation costs is desirable.
During clearwater drilling, water is pumped downhole through the drill string to the drill bit, and is returned to the surface as a slurry, carrying cuttings and suspended solids. The slurry is then clarified and the resulting clear water is recycled for use in further drilling. At various stages of drilling, different drilling fluid compositions are required, and it is preferable that each fluid is similarly clarified and recycled to conserve resources and minimize environmental damage.
In typical drilling systems, used drilling fluid is clarified using a combination of screening, shakers, hydrocyclones, settling tanks, centrifuges, and flocculating agents. In systems incorporating primarily gravity-based settling means, a horizontal settling tank system may be used to provide a long flowpath for passage of the fluid, maximizing fluid retention time within the system and providing sufficient opportunity for solids to settle from the fluid. Once the fluid stream has passed through the entire system, solids have settled at the bottom of the tank, and the drilling fluid has been sufficiently clarified that it may be reused downhole.
Over time, settled solids build up in the tank compartments and must be removed to maintain the quality of the clarified output fluid. Various prior art systems employ solids conveying systems to transport solids from the bottom of the settling compartments to the end or top of the tank for disposal exterior the system. For example, U.S. Pat. No. 6,059,977 teaches the use of an auger system for conveying the solids through the bottom of each tank compartment to an outlet port; U.S. Pat. No. 6,863,809 teaches pumping of solids through collection pipes to a centrifuge; and U.S. Pat. No. 6,391,195 teaches the use of a drag conveyor for transporting solids from the bottom of the solids tank upwardly to an outlet higher in elevation than the tank contents. Such solids removal systems involve mechanical parts which require maintenance and may also become clogged or jammed with solids during use. Moreover, the prior art tank systems are difficult to empty and clean following completion of drilling.
An additional objective of some fluid clarification systems is to concentrate or thicken the solids stream for disposal directly on the ground on-site. This generally requires a series of centrifuges to dewater the solids stream, as selective removal of thickened solids from the settling tank is not possible. As centrifuges are expensive to operate, requiring regular maintenance as well as consuming significant amounts of energy, it is desirable to minimize the use of centrifuges at the drilling site by improving the concentration of the solids stream during settling.
Further, in horizontal settling tank systems, solids accumulate in the initial settling compartment more rapidly than in each subsequent settling compartment. Moreover, the initial settling tank will generally accumulate larger, heavier solids that settle easily from the fluid, while the final compartment will generally accumulate finer solid particles, at a slower rate. Thus, solids should generally be emptied from the initial compartment more frequently than from subsequent compartments. Such selective solids removal is not possible using the solids conveying means of current horizontal settling tank systems.